Stem cells comprise a unique population of cells that divide asymmetrically to balance self-renewal with differentiation. The daughter of a stem cell undergoes transit-amplification, where a tightly controlled number of mitotic divisions expand the cell pool before terminal differentiation. However, the mechanisms regulating this process remain poorly understood. As in mammals, Drosophila spermatogenesis also utilizes transit-amplification to produce spermatocytes from germline stem cells. Recent work in the DiNardo lab suggests that the BMP pathway is necessary for transit amplifying (TA) spermatogonial cells to transition into spermatocyte differentiation. Additionally, counting mutants, which increase (count up; ctu) or decrease (count down; cdn) the number of TA divisions by one has been identified. Cross species transplantation experiments will test the model that a counting mechanism intrinsic to TA germ cells monitors division number. Cloning cdn and positioning ctu and cdn in the circuit regulating the TA gonia to spermatocyte transition will further examine if somatic cells signal to the germ cells to trigger the shift to spermatocyte differentiation. Such feedback may provide for the coordinated differentiation of somatic and germline cells.